The present invention relates generally to bandgap reference circuits, and in particular to low voltage bandgap circuits with trimming and curvature correction methods.
The output voltage of a bandgap reference circuit is as follows: EQU V.sub.ref =V.sub.be +(K.times..DELTA.V.sub.be)
where:
K is a gain factor set by a resistor ratio, and PA1 Vbe is a bipolar transistor base-emitter voltage drop.
The output of a bandgap circuit is typically around 1.2 v. Improved bandgap circuits include an operational amplifier (opamp) to improve power supply rejection. The opamp is connected in a feedback loop with a closed-loop gain of K. This gain amplifies any inherent opamp offset voltage, which then appears as an error voltage at the output of the bandgap circuit. To reduce the error voltage contribution due to the opamp output-referred offset, different techniques are used to lower the amount of the required closed-loop gain. For a fixed output voltage (V.sub.ref), the closed-loop gain (K) may be reduced if the value of the .DELTA.V.sub.be term is made larger. Two different methods are used to generate the .DELTA.V.sub.be term: (1) differing amounts of currents are forced through two identical diode connected bipolar transistors or (2) an equal amount of current is forced through two bipolar transistors having different sizes. Both of these methods require adding a large number of transistors to obtain either (1) an appreciable current differential, or (2) an appreciable transistor size differential.
A more efficient way of increasing the value of the .DELTA.V.sub.be term is to connect another diode connected bipolar transistor in series with each one of the diode connected bipolar transistors that generate the .DELTA.V.sub.be term. This way the .DELTA.V.sub.be term can be, for example, doubled when two diodes are stacked, or tripled when three diodes are stacked. Stacking the bipolar transistors however, creates two potential problems. First, the resulting output voltage of the reference circuit is no longer 1.2 v, but a higher multiple of that voltage (i.e. 2.4 v, or 3.6 v) (see IEEE JSSC, pg. 896, FIG. 6). Secondly, such stacking limits the voltage range in which the bandgap circuit can properly operate. This is so for several reasons. First, the power supply cannot be lower than the output voltage. If the output voltage is not 1.2 v but rather a multiple of 1.2 v, this can be a problem. Secondly, the portion of the circuitry that generates the .DELTA.V.sub.be (or rather, some multiple of .DELTA.V.sub.be) needs more headroom to operate when stacked. Therefore, low voltage operation of the bandgap circuit is sacrificed to reduce the error voltage due to opamp offset.
In bandgap circuits, it is often necessary to fine tune the circuit to obtain highly accurate reference voltages. This fine tuning is accomplished by trimming the resistor ratio to change the K factor in very small steps. To obtain fine resolution with resistor trimming, however, is costly in several respects. First, provisions for a resistor bank and the corresponding switching circuitry consumes valuable silicon area. Secondly, the switching mechanism in resistor trimming adds more complexity and places additional requirements on the circuit. If, for example, transistor switches are used to switch trimming resistors in and out, transistor resistances would have to be accounted for, which may result in very large switch transistors and/or very large resistors. In addition, decoding may be required for the switches, which would use up more area. Other trimming techniques require a silicon fabrication process that supports zener zapping (to short or insert resistors) and fusible metal. links (to remove resistors). Not all CMOS processes can support zeners. Existing resistor trimming techniques are therefore, costly and inflexible.
Another critical feature of a bandgap circuit is its temperature performance. Bandgap circuits are designed to yield an output voltage with zero temperature coefficient (TC), at a particular temperature TO (e.g. 25.degree. C.). However, the bandgap output voltage drops as temperature departs from TO in either direction. Existing curvature correction schemes involve either a specially designed architecture, or require adding positive TC resistors in series with the emitter or a base of some of the .DELTA.V.sub.be bipolar transistors. This also adds to circuit complexity and cost.